Digester cover left-in-place ballast ring

ABSTRACT

A sludge digester including a vessel and floating cover. The vessel includes a sidewall and an interior volume configured to receive and contain sludge. According to one embodiment, the cover comprises a frame structure that is constructed and arranged to form a skirt member formed at a periphery of the cover and extending downwardly into the vessel, and a continuous ballast ring attached to a lower portion of the skirt member and configured to form a trough member with an interior surface of the skirt member. The sludge digester may also include a guide system coupled to the sidewall and the skirt member and configured to allow vertical displacement of the cover with change in volume of at least one of a gas and a sludge contained in the vessel beneath the cover.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority under 35 U.S.C. § 119(e) to U.S.Provisional Patent Application Ser. No. 62/508,045, filed on May 18,2017, titled DIGESTER COVER LEFT-IN-PLACE BALLAST RING FORM, the entiredisclosure of which is hereby incorporated herein by reference for allpurposes.

FIELD OF THE DISCLOSURE

Aspects relate generally to covers for sludge digesters, and morespecifically to floating covers.

SUMMARY

Aspects and embodiments are directed to a floating cover for a sludgedigester. In accordance with an embodiment of the present inventionthere is provided a sludge digester. The sludge digester comprises avessel having a sidewall and an interior volume configured to receiveand contain sludge and a cover configured to extend over the interiorvolume of the vessel and seal the interior volume from the atmosphereand trap a gas generated by sludge contained in the vessel. The covercomprises a frame structure constructed and arranged to form: a skirtmember formed at a periphery of the cover extending downwardly into thevessel, and a continuous ballast ring attached to a lower portion of theskirt member and configured to form a trough member with an interiorsurface of the skirt member, and a guide system coupled to the sidewalland the skirt member and configured to allow vertical displacement ofthe cover with change in volume of at least one of the gas and thesludge contained in the vessel beneath the cover.

In accordance with some aspects of the sludge digester, the troughmember has a closed bottom end and an open top end.

In accordance with some aspects of the sludge digester, the troughmember includes first and second sidewalls extending from the closedbottom end, one of the first and second sidewalls formed from theinterior surface of the skirt member.

In accordance with some aspects of the sludge digester, the framestructure includes a series of frame sections, each frame sectionforming one of a chord or a curve that connect to each other to define agenerally circular outer perimeter of the cover.

In accordance with some aspects of the sludge digester, each curvedframe section has concavely curved radially outer and inner walls andeach chordal frame section has a concavely curved radially outer walland a planar inner wall.

In accordance with some aspects of the sludge digester, each framesection includes at least one brace element extending between andcoupled to the outer and the inner walls.

In accordance with some aspects of the sludge digester, the troughmember is configured to hold a ballast material.

In accordance with some aspects of the sludge digester, the ballastmaterial is a continuous solid concrete ring.

In accordance with some aspects of the sludge digester, the coverfurther includes a gas dome disposed in the center of the cover andextending axially downward below a lower portion of the skirt member.

In accordance with some aspects of the sludge digester, the framestructure is further constructed and arranged to form a series of radialbeams, each radial beam extending from an upper portion of the skirtmember to the gas dome.

In accordance with some aspects of the sludge digester, the coverfurther comprises at least one cover plate attached to the framestructure and configured to extend between adjacent radial beams.

In accordance with some aspects of the sludge digester, the framestructure is constructed from steel.

In accordance with some aspects of the sludge digester, the framestructure further includes an inlet opening for delivering ballastmaterial to the trough member.

In accordance with another embodiment of the present invention there isprovided a method of forming a cover of a sludge digester, where thesludge digester has a vessel with an interior volume. The methodcomprises constructing a series of frame structure sections, eachsection including an outer wall and an inner wall spaced apart from theouter wall, the outer wall and the inner wall extending vertically froma horizontal base portion to form a trough member, and joining theseries of frame structure sections to each other to form a continuousring, the continuous ring forming a perimeter of the cover and sized tofit within an inner diameter of the vessel.

In accordance with some aspects, the method further includes filling atleast a portion of the trough member with ballast material to form acontinuous solid ring.

In accordance with some aspects, filling is performed inside theinterior volume of the vessel.

In accordance with some aspects, constructing each section includesattaching the horizontal base portion to the outer wall.

In accordance with some aspects, the outer wall is constructed to extendvertically above the inner wall and the method further includesattaching a roof structure to an upper portion of the outer wall.

In accordance with another embodiment of the present invention there isprovided a form for constructing a cover for a sludge digester. The formcomprises a first metal panel shaped to form a bottom panel and a firstside panel of a trough member, the first side panel configured to formone of a curved shape or a planar shape, the trough member sized andshaped to be and to hold a continuous ring of ballast material and toconnect to a periphery of the cover.

In accordance with some aspects, the form further includes a secondmetal panel attached to the bottom panel, the second panel forming asecond side panel of the trough member and configured to form a curvedshape.

Still other aspects, embodiments, and advantages of these exampleaspects and embodiments, are discussed in detail below. Moreover, it isto be understood that both the foregoing information and the followingdetailed description are merely illustrative examples of various aspectsand embodiments, and are intended to provide an overview or frameworkfor understanding the nature and character of the claimed aspects andembodiments. Embodiments disclosed herein may be combined with otherembodiments, and references to “an embodiment,” “an example,” “someembodiments,” “some examples,” “an alternate embodiment,” “variousembodiments,” “one embodiment,” “at least one embodiment,” “this andother embodiments,” “certain embodiments,” or the like are notnecessarily mutually exclusive and are intended to indicate that aparticular feature, structure, or characteristic described may beincluded in at least one embodiment. The appearances of such termsherein are not necessarily all referring to the same embodiment.

BRIEF DESCRIPTION OF DRAWINGS

Various aspects of at least one embodiment are discussed below withreference to the accompanying figures, which are not intended to bedrawn to scale. The figures are included to provide an illustration anda further understanding of the various aspects and embodiments, and areincorporated in and constitute a part of this specification, but are notintended as a definition of the limits of any particular embodiment. Thedrawings, together with the remainder of the specification, serve toexplain principles and operations of the described and claimed aspectsand embodiments. In the figures, each identical or nearly identicalcomponent that is illustrated in various figures is represented by alike numeral. For purposes of clarity, not every component may belabeled in every figure. In the figures:

FIG. 1 is a partial schematic and cross-sectional view of a sludgedigester with a cover;

FIG. 2 is an enlarged and partially exploded perspective view of thesludge digester shown in FIG. 1;

FIG. 3 is a cross-sectional view of an example of a portion of a coverpositioned within a vessel of a sludge digester in accordance withaspects of the invention;

FIG. 4 is an enlarged view of a trough member portion of the cover shownin FIG. 3;

FIG. 5 is a cross-sectional view of an example of a trough member inaccordance with aspects of the invention;

FIG. 6 is a cross-sectional view of another example of a portion of acover positioned within a vessel of a sludge digester in accordance withaspects of the invention;

FIG. 7A is a top view of one example of a frame structure for a cover inaccordance with aspects of the invention;

FIG. 7B is a top view of another example of a frame structure for acover in accordance with aspects of the invention;

FIG. 8 is a top view of a section of the frame structure shown in FIG.7A;

FIG. 9 is a top view of a section of the frame structure shown in FIG.7B;

FIG. 10 is a top perspective view of the sludge digester shown in FIG.1; and

FIG. 11 is an enlarged and partially exploded perspective view of thestructure shown in FIG. 10, with portions removed.

DETAILED DESCRIPTION

Wastewater treatment processes use digestion tanks for anaerobicallytreating wastewater sludge. During the anaerobic process, gas, a mixtureof primarily methane and carbon dioxide, is given off and collected tobe used as fuel for performing treatment processes in the system, suchas heating the sludge mixture. Some systems employ a floating gas holderpositioned above the sludge which collects the gas and provides acontrollable downward force on the gas, which results in the gas beingpressurized and useable as fuel.

Conventional floating covers are constructed on-site from steel andconcrete materials. One of two construction and assembly methods aretypically used. The first involves use of separate concrete ballastblocks that are manufactured off-site, delivered, and then liftedindividually into the digester tank and positioned on a steel structurethat forms the cover. The second involves use of a temporary form thatis constructed on-site. Concrete is poured into the temporary form, andonce cured, functions as the ballast for the cover.

The use of separate concrete ballast blocks creates several potentialproblems. One is the potential for the blocks to fall into the digesteras the cover ages and the materials deteriorate. Another is that theseparate ballast blocks pose a safety risk during the installation phaseof the system. Cranes are used to lift the blocks, which can weighseveral tons, and place them onto the steel structure inside thedigester that is intended to hold them. This requires workers to be inthe tank during lifting, since the crane operator is unable to see theblocks once they are lowered into the digester. This creates a “blindlift” operation, which poses safety risks for the workers in the tank.

The use of temporary forms built on-site for forming ballast structuresalso presents several problems. One is that the temporary formwork isconstructed from plywood and has to be built on-site, and in someinstances, built inside the digester. The temporary plywood form alsohas to be built to specified dimensions that may differ from one site toanother. The form has to be re-built when these dimensions are notimplemented correctly. Once the concrete cures, the forms also have tobe removed, and are thrown away as waste. The on-site form-makingprocess thus makes for difficult work and is time consuming.

In accordance with certain embodiments, the use of a continuous ballastring is used for a floating digester cover. The continuous ballast ringincludes ballast material that forms a single structure that can beconstructed from concrete and is poured in one operation. The ballastmaterial for the ballast ring therefore does not have any seams or gaps.The continuous ballast ring provides advantages over the use of separateballast blocks in that it provides rigidity to the cover and minimizesthe potential of concrete falling into the digester as the ballastmaterials deteriorate. The risk to operators inside the tank during theinstallation phase is also reduced since the ballast material can bedelivered and cured within the digester itself.

According to some embodiments, a ballast ring, also referred to as a“ballast pick-up ring” for use in a floating digester cover isconfigured to provide additional weight to meet desired operatingpressures and to allow operation of pressure/vacuum relief safety valveswhen the digester cover is filled with gas. The ballast ring of thepresent invention in some embodiments is constructed from a steel formthat is designed and fabricated with the digester cover. The steel formis designed to attach to a rim skirt of the digester cover, which allowsthe installing contractor to fill the form with flowable fill concrete.The steel ballast ring is therefore integral to the rim skirt and iserected with the rim skirt and left in place after being filled withconcrete or other ballast material. The steel form provides additionalstructure to the digester cover, produces a consistent and preciseresult since it can be designed with optimum dimensions prior toinstallation, reduces the amount of time needed for an installingcontractor to erect the cover, and provides a safer work environment forthe installation crew. The steel form also saves time on dismantling anddisposing of the temporary concrete formwork that is used inconventional processes.

Referring to FIGS. 1 and 2, a sludge digester, generally indicated at100 is shown. The sludge digester 100 includes a vessel 105 having asidewall 107 and an interior volume 109. A cover 110 includes a skirtmember 112 and a continuous ballast ring 114 comprising ballast material135, such as concrete, is attached to the skirt member 112. The sludgedigester 100 also includes a guide system 145 coupled to the sidewall107 and the skirt member 112 that is configured to allow verticaldisplacement of the cover 110 with change in volume of at least one ofgas and sludge contained in the vessel 105 and beneath the cover 110.The vessel 105 also includes a series of corbels 111 that projectinwardly toward the center of the interior volume 109 of the vessel 105.Initially, the operating pressure of the digester is such that a portionof the skirt member 112 and ballast ring are submerged in the sludge andrest on the corbels 111, which may be sized to be wider than the ballastring. One or more hanger straps 138 may be used to attach the ballastring 114 to the skirt member 112. The cover 110 may also include a roofstructure 125 that attaches to an upper portion of the skirt member 112.The ballast ring 114 shown in FIGS. 1 and 2 does not show the steelballast form that is described in further detail below.

According to one embodiment, a cover 110 is disclosed that is configuredto extend over the interior volume 109 of the vessel 105 and seal theinterior volume 109 from the atmosphere and trap gas generated by sludgecontained in the vessel 105. One example of a portion of a cover 110 isshown in the cross-sectional view shown in FIG. 3. The cover 110includes a frame structure 115 that is constructed and arranged to forma skirt member 112, also referred to as a “rim skirt” or “side skirt,”and a continuous ballast ring 114 attached to a lower portion of the rimskirt 112. The skirt member 112 is formed at a periphery of the coverand extends downwardly into the vessel 105. The continuous ballast ring114 is configured to form a trough member 116 with an interior surface113 of the skirt member 112, an enlarged view of which is shown in FIG.4.

The trough member 116 may be constructed from one or more pieces ofmetal material, such as steel (e.g., sheet steel), and includes a closedbottom end or base portion 119, and first and second sidewalls extendingfrom the closed bottom end 119, where one of the first and secondsidewalls is formed from the interior surface 113 of the skirt member112. The other sidewall of the tough member 116 is formed by inner wall118. The skirt member 112 therefore forms an outer wall of the troughmember 116 and the inner wall 118 is spaced apart from the outer wall,with both the inner wall and outer wall extending vertically from thehorizontal base portion 119. The frame structure 115 may also include atleast one brace element 132 that extends between and is coupled to theouter wall 112 and inner wall 118 of the trough member 116.

As shown in FIG. 3, the trough member 116 is configured to hold aballast material 135. Non-limiting examples of ballast materials includedense heavy materials such as concrete. Examples of concrete that may besuitable for ballast materials as described herein include concretehaving a nominal compressive strength of about 2500 psi to about 4000psi, when cured. In some embodiments, the ballast material 135 may beconcrete having a compressive strength of about 4000 psi. Concretematerial can be poured into the trough member 116 once sections 120 ofthe frame structure 115 have been connected to each other (described infurther detail below). When cured, the concrete forms a continuous solidconcrete ring.

FIGS. 4 and 5 show examples of two cross-sectional views of troughmember 116. The trough member 116 may be constructed in part from a form150. According to one embodiment, the form 150 is constructed from ametal panel or sheet that is shaped to form the bottom panel 119 and afirst side panel 118 (i.e., inner wall) of the trough member 116. Theexamples shown in FIGS. 4 and 5 shows the metal panel being a singlemetal panel, but in other embodiments, the base portion 119 and firstside panel 118 may be formed from separate metal panels and then joinedtogether via an attachment mechanism (e.g., welding).

As discussed further below, the frame structure 115, including thetrough member 116 of the ballast ring 114, may be formed from metalmaterials such as steel, including stainless steel, but other materialsare also within the scope of this disclosure, including compositematerials such as fiber reinforced polymer (FRP) materials. Forinstance, various types of fibers, including glass, carbon, nylon,polyester, and/or Kevlar® can be included in a polymer matrix to formstructural members as described herein.

The form 150 may be attached to the skirt member 112 using an attachmentmechanism 134. In the examples shown in FIGS. 4 and 5, the horizontalbase portion 119 is attached to the skirt member 112. In one embodiment,the attachment mechanism 134 includes one or more mechanical fasteners,such as bolts, gaskets, screws, washers, rivets and the like. Theattachment mechanism 134 shown in FIGS. 4 and 5 is a mechanical type offastener. In another embodiment, the attachment mechanism 134 isaccomplished via welding, e.g., a welded seam or other welded attachmentconfiguration. Welding can be performed on-site or in off-site,depending on the part of the frame structure being constructed and thesize of the overall structure. In some instances, both a mechanicalfastener and welding techniques may be used for joining portions of theframe structure 115. Once attached, the form 150 includes the metalpanel of the skirt member 112, which extends from the bottom panel 119and forms a second side panel with the first side panel (118) forconfining and forming the shape of the ballast material 135.

According to one embodiment, the trough member 116 has an open top end.The top end is disposed opposite the closed bottom end 119 and may beconfigured to be open for purposes of introducing ballast material 135to the trough member 116. The closed bottom end 119, inner wall 118, andportion of the interior 113 of the skirt member 112 that comprise thetrough member 116 provide rigid support to uncured, pourable, fluid-likeballast material 135 while the ballast material cures into a dense,solid material. The “openness” of the top end of the trough member 116may be “broken” up by brace elements 132. The brace elements 132 may beformed from a rod-like metal material, such as rebar. In someembodiments, the brace elements 132 extend between and are attached tothe skirt member 112 and inner wall 118. In other embodiments, the braceelement 132 may extend between the skirt member 112 and inner wall 118but may not be attached to either structure. The brace elements 132 mayprovide structural support to the trough member 116 and/or ballastmaterial 135. In some embodiments, the ballast material 135 may beconfigured to have a sloped top 136, as shown in FIG. 5. In certaininstances the sloped top 136 can be formed after the ballast material135 has been poured and has started to cure or otherwise “set” such thatworkers can use a temporary form or manually form with a trowel thefinal sloped shape. The sloped top 136 of the ballast material 135 mayfunction to allow the cover 110 to more easily move vertically (i.e.,“rise”) through sludge contained in the interior volume 109 of thevessel 105 than a ballast material top that is configured to behorizontally planar. The sloped top 136 may also function to allowrunoff of sludge from the ballast ring, which may reduce the rate ofdeterioration or wear of the ballast material 135.

Referring now to FIG. 6 another cross-sectional view of the cover 110 isshown. In this view, a hanger strap 138 extends from the skirt member112 to the inner wall 118 of the trough member 116, which in thisparticular view does not include ballast material. The hanger strap 138functions to add structural support to the frame structure 115 and formsanother means of attachment for the trough form 150 to attach to theskirt member 112.

The frame structure 115 includes a series of frame sections 120, asshown in the top views of FIGS. 7A and 7B. Each frame section 120 isconstructed identically or near-identically and is configured to attachto another frame structure section 120. Each frame structure section 120(also referred to herein as simply “frame section” or “section”) isconfigured to form one of a chord (e.g., FIG. 7A) or a curve (e.g., FIG.7B) that connect to each other to define a generally circular outerperimeter of the cover 110. The outer perimeter of the cover is sized tofit within an inner diameter of the sidewall 107 of the vessel 105.According to the examples shown in FIGS. 7A and 7B, 16 frame structuresections 120 are joined together to form the outer perimeter of thecover. It is to be appreciated that other embodiments may include lessthan or more than 16 frame structure sections, with the number of framestructure sections depending on the specific application, the size ofthe digester, and the desired dimensions of the ballast ring.

In FIG. 7A, each frame structure section 120 forms a chord, and FIG. 8is a top view of one chordal section 122. The chordal frame section 122is configured to have a concavely curved radially outer wall 112 (i.e.,skirt member 112) and a planar inner wall 118 a. Each chordal section122 also includes brace elements 132 that are spaced at regularintervals and extend between the outer wall 112 and inner wall 118 a. Asnoted above, the example shown in FIG. 7A includes 16 chordal framesections 122, but it is to be appreciated that the frame structure maybe divided into other numbers of chordal sections that are less than orgreater than 16. The chordal sections (and curved sections describedbelow) may be sized and shaped such that when they are joined togetherthe outer perimeter of the structure forms a continuous ring that fitswithin the interior diameter of the vessel.

In FIG. 7B, each frame structure section 120 forms a curve, and FIG. 9is a top view of one curved section 124. The curved frame section 124 isconfigured to have a concavely curved radially outer wall 112 (i.e.,skirt member 112) and a concavely curved radially inner wall 118 b. Eachcurved section 124 also includes brace elements 132 that are spaced atregular intervals and extend between the outer wall 112 and the innerwall 118 b. The curve of the skirt member 112 and the inner wall 118 bmay be configured such that the distance between the skirt member 112and the inner wall 118 b is maintained at an even space (i.e., the samedistance apart).

As shown in FIGS. 8 and 9, the skirt member 112 that forms the outerwall of each frame structure section 120 is configured to follow thecontour of the curve of the interior of the sidewall 107 of the vessel105. In some embodiments, the skirt member 112 may be spaced apart fromthe sidewall 107 by a predetermined distance to allow the cover 110 tofreely move vertically within the vessel 105 as sludge is introduced toand contained in the vessel 105 and gas is generated from the sludgeduring the digestion process.

The individual frame structure sections 120 may be joined to each otherusing any one of a number of different methods. According to oneembodiment, the sections 120 are joined using welding techniques. Inother embodiments, the sections are joined using mechanical attachmentmethods, such as bolts and gaskets. In some instances, each section 120is constructed outside of the interior volume 109 of the vessel 105, andjoining the series of frame structure sections 120 is performed insidethe interior volume 109 of the vessel 105. For example, the trough form150 and skirt member 112 may be prefabricated either on-site oroff-site. Likewise, the trough form 150 may be attached to the skirtmember 112 either off-site or on-site. As attached, these two componentsof the frame structure 115 may form each frame structure section 120,which may then be individually placed into the vessel 105 where they arethen joined together to form a continuous ring. In some instances, theframe structure sections may be joined together outside the vessel, andthen lowered in via a crane.

According to one embodiment, the frame structure 115 may be constructedfrom one or more metal materials such as steel (e.g., carbon steel),including stainless steel, and in some instances may be constructed fromaluminum. In other embodiments the frame structure 115 may beconstructed from one or more other types of materials, such as compositematerials, including carbon reinforced metal composites or other type ofcomposites such as FRP materials. The material or materials used to formthe frame structure 115 may be any material that is suitable for meetingthe structural and mechanical requirements of the cover and capable ofwithstanding the environmental conditions experienced by the cover. Theframe structure 115 may include panels or sheets of metal material, aswell beams, bars, rods, or any other structural element that functionsto provide a frame structure as described herein. The frame structure115 may also include attachment mechanisms, such as attachment mechanism134 that is used to attach the trough form 150 to the skirt member 112,as well as other attachment mechanisms disposed at joining locations ofthe structure.

The specific dimensions, e.g., length, width, thickness of materialforming the frame structure 115, depend on the specific applicationand/or dimensions of the vessel. In some embodiments, the panels formingthe skirt member 112 and trough form 150 are constructed from steel thatis at least ¼ inch thick. The thickness of the steel may depend on theparticular design and may be as thick as deemed necessary to comply withthe structural and mechanical requirements of the cover. According tosome embodiments, the brace elements 132 are formed from metal rodmaterial having a ½ inch diameter.

In certain embodiments, the trough member 116 is sized or otherwiseconfigured to accommodate expansion and contraction of the ballastmaterial 135 and/or frame structure material. For instance, joints wheremating pieces or sections of frame structure meet may include channels,gaps, or an elastic material to allow adjoining materials to expand orcontract. These joints may be hermetically sealed via other structuralportions of the joint.

According to some embodiments, the trough form 150 is configured toattach to a lower portion of the skirt member 112. Positioning theballast at a lower portion of the skirt member 112 functions to lowerthe metacenter of the cover, as well as add stability to the cover. Insome embodiments, the bottom end 119 of the form 150 aligns with thebottom edge of the skirt member 112, as shown in FIGS. 4 and 5. Thisconfiguration allows for at least a portion of the bottom end 119 of thetrough member 116 to “rest” on the corbel structures 111. The examplesshown in the figures include individual corbels 111, but it is to beappreciated that the corbels may be constructed as a continuousstructure that forms a support “shelf” around the interior of the vesselsidewall 107. In some embodiments, the bottom end 119 of the trough form150 may attach further up from the bottom edge of the skirt member 112,such that a “lip” of the skirt member 112 extends down below the bottomend 119.

As described above, the frame structure 115 comprises the skirt member112 and the ballast ring 114. The trough form 150 of the continuousballast ring 114 is not removed once the ballast material 135 is addedand therefore contributes to the weight of the ballast ring 114. Gasproduced by the sludge contained in the vessel 105 is pressurized by theweight of the cover, including the ballast ring 114. The pressurized gascan then be removed via the gas dome 140 and/or safety valve 148 (seeFIG. 10 for example of safety valve 148).

In one embodiment, the frame structure 115 includes one or more inletopenings for delivering ballast material 135 to the trough member 116inside the vessel. The inlet opening(s) may function to provide aconnection for fill hoses that are used to deliver ballast material tothe trough member 116. The inlet opening(s) may provide structuralsupport for the fill hoses and also allow for maneuverability within thevessel 105 during the filling process. One example of such an inletopening is shown in FIG. 3 at 130. The inlet opening 130 may bepositioned at one or more locations in the frame structure 115. Theexample shown in FIG. 3 has the inlet opening 130 formed within aceiling plate 152 of the roof structure 125 (described in further detailbelow). In this position, one or more fill hoses can be connected tothis location (e.g., a top threaded portion of the inlet opening 130)from the concrete delivery vehicle, and then one or more additional fillhoses can be attached to the inlet opening (e.g., a bottom threadedportion of the inlet opening 130) that can be used for targeted deliveryto the trough member 116. Additional inlet openings may be formed inother locations, for instance in the cover plate 128 of the roofstructure shown in FIG. 3.

In accordance with at least one embodiment, the frame structure 115further includes a roof structure 125. The roof structure 125 isconfigured to keep atmospheric air from entering the digester and mixingwith the generated biogas. In some embodiments, the roof structure 125is attached to an upper portion of the skirt member 112, which extendsvertically above the inner wall 118 of the trough member 116.

One non-limiting example of a roof structure 125 is shown in FIGS. 10and 11. For example, a series of radial beams 126, as shown in FIGS. 10and 11, extend from an upper portion of the skirt member 112 to the gasdome 140 disposed in the center of the cover 110. The radial beams 126may curve upward into an arc or arch, which increases the volume ofspace beneath the cover for storing gas generated by the sludge. One ormore cover plates 128 attach to the frame structure 115 and areconfigured to extend between adjacent radial beams 126. In someinstances, a single cover plate 128 may be used that is attached to anunderside (or alternatively, a top side) of the radial beams 126. Asshown in FIG. 2, according to some embodiments, the upper portion of theskirt member 112 may be attached to a peripheral structure 129 (alsoreferred to as an outer thrust ring) that extends around the topperimeter of the cover 110. For instance, the example shown in FIG. 2includes a skirt member 112 where the upper edge is bent to form a“shelf” that is attached to the peripheral structure 129. As indicatedin FIG. 2, the radial beams 126 may attach to the peripheral structure129. The peripheral structure 129 may be constructed from the samematerial as the frame structure 115, such as steel, and may beconfigured to ensure that no excessive loading is transferred to thesidewall 107 of the vessel 105. This may allow for the cover 110 to moreeasily move vertically within the vessel 105. One or more of the coverplates 128 may include an inlet opening 130, as indicated in FIGS. 1 and10, for assisting in delivering ballast material.

Another example style of roof structure 125 is shown in FIG. 3 (and FIG.6, although not explicitly labeled) which includes a truss style roof.As shown in FIG. 3, roof trusses 155 form a structural frame for theroof structure. Cover plates 128 and ceiling plates 152 attach to thetop and bottom of the trusses 155 respectively, with an air space orattic space 154 formed in between the cover plates 128 and ceilingplates 152. The attic space 154 may function to protect the roof trusses155 from corrosion and may also provide insulation for the sludgecontained in the vessel.

Other types of roof structures are also within the scope of thisdisclosure. In addition, other structural elements may also be includedin the roof structure, such as purlins that connect between the mainstructural beams/ribs and add lateral bracing, rigidity and support.

According to at least one embodiment, the form 150 may be attached tothe skirt member 112 to form a frame structure section 120, and then theframe structure sections 120 may be attached to each other. The roofstructure 125 may then be attached to the joined frame structuresections. For instance, the radial beams 126 may be attached to theskirt member 112 after the frame structure sections 120 are positionedand joined to one another. The cover plate(s) 128 may be added after theradial beams 126 are attached to the skirt member 112.

Depending on the size of the cover, one or more portions of the framestructure 115 may be formed inside or outside the vessel 105. Forinstance, in some instances, the frame structure sections 120 may beattached to one another and the roof structure 125 attached to the framestructure sections 120 external to the vessel 105. Then the entire framestructure 115 may be lowered into the vessel via a crane, and onceinside the vessel 105, the ballast material 135 may be delivered to theform 150. In other instances, the frame structure sections 120 may beconstructed external to the vessel and then attached to each otherinside the vessel. The roof structure 125 may then be attached.

According to some embodiments, the gas dome 140 is configured to form aliquid seal with the sludge contained in the vessel 105, which traps gasproduced by the sludge and contributes to the ability of the cover 110to float. For example, the gas dome 140 is disposed in the center of thecover 110 and may extend axially downward below a lower portion of theskirt member 112. The ballast ring 114 formed from the trough member 116and ballast material 135 is initially submerged within sludge containedin the vessel 105. As the volume of gas generated from the digestingsludge increases, a buoyant force is exerted by the sludge upon theballast ring 114 according to Archimedes principal. The ballast ring 114begins to rise through the sludge, and eventually “floats” on a surfaceof the sludge such that the ballast ring 114 is partially submergedduring normal operation. The pressure relief or safety valve 148 isconfigured such that the gas pressure under the cover 110 does notexceed the static weight of the cover 110. Keeping the gas pressurebelow this threshold gas pressure allows for the ballast ring 114 toremain partially submerged, which adds to the stability of the cover110. If the threshold gas pressure under the cover is exceeded, theballast ring 114 becomes unsubmerged and floats, which makes the cover110 unstable and may allow for gas contained under the cover to escapeto the atmosphere.

When concrete is used as the ballast material 135, the concretetypically has a density of about 150 pounds per cubic foot. A cubic footof concrete in a submerged condition in sludge having a specific gravityof about 1.0 has an effective weight which is reduced by the weight of acubic foot of sludge (about 62.4 pounds per cubic foot). Thus, in asubmerged condition, one cubic foot of solid concrete ballast exerts adownward force of about 87.6 pounds. The weight of the form 150 may alsobe factored into this calculation, but the weight of the form isnegligible when compared with the weight of the ballast material.

When the ballast ring 114 emerges from the sludge, the effective weightof the concrete is its normal density, i.e., about 150 lbs/ft³. Thus,the total weight of the cover is significantly greater when the ballastmembers are in an emerged condition than when the ballast members are ina submerged condition. This creates a gas pressure differential betweenthe submerged and emerged positions of the ballast. Typically, theoperating pressure of the digester is that of the ballast when it israised from the corbels but still in a submerged condition. When theballast ring 114 fully emerges from the sludge, the pressure generatedis usually at or above the relief valve settings so that typically theballast ring 114 is never fully emerged from the sludge without therelief valves on the cover relieving the pressure of the gas. Theballast ring 114 is therefore sized to provide the required dead weightto meet the desired operating pressure. The ballast ring 114 may also besized based on a desired increase in pressure (threshold pressure) thatoccurs from the ballast becoming unsubmerged.

Referring back to FIGS. 1 and 2, the sludge digester 100 according to atleast one embodiment also includes a guide system 145. In someembodiments, the guide system 145 is coupled to the sidewall 107 of thevessel 105. The floating cover 110 has a high center of gravity and ismainly supported on gas generated by the digesting sludge contained inthe vessel. These factors make the cover 110 unstable, and therefore theguide system 145 may not only function to allow vertical displacement ofthe cover 110, but may also function to provide stability to the cover110 throughout its range of travel. The example shown in FIGS. 1 and 2is configured as a vertical guide system 145, but other configurationsare also within the scope of this disclosure, such as spiral guidesystems. The guide system 145 may include a rolling-type of apparatusthat connects to the exterior of the skirt member 112 and moves within atrack assembly mounted to the sidewall 107.

The vessel 105 may include a circular vertical sidewall 107 that isconfigured as a cylinder. Other configurations for the vessel are alsowithin the scope of this disclosure. For instance, the vessel may beconfigured as a polygon with a plurality of planar walls. The examplesdiscussed herein utilize sludge as an example of a gas-generatingmaterial contained within the vessel, but it is to be appreciated thatany material that generates gas may be stored or otherwise placed in thevessel. For instance, any material suitable for anaerobic digestion maybe introduced to the vessel 105, including wastewater from agricultural,municipal, and industrial processes, as well as waste such asmicroorganisms, biomass, biological waste, slurries, etc.

The floating cover described herein offers many advantages over floatingcovers that do not include the trough form frame structure. The floatingcover of the subject invention provides additional rigidity to the coverstructure, reduces costs by removing the need to build temporarypreforms for forming the ballast material, and increases contractorsafety by eliminating the need to do a blind lift operation of ballastmaterial into the digester tank. The continuous ring configuration forthe ballast structure also prevents the chance of loose blocks fallinginto the digester tank and damaging its internal structure.

The aspects disclosed herein in accordance with the present invention,are not limited in their application to the details of construction andthe arrangement of components set forth in the following description orillustrated in the accompanying drawings. These aspects are capable ofassuming other embodiments and of being practiced or of being carriedout in various ways. Examples of specific implementations are providedherein for illustrative purposes only and are not intended to belimiting. In particular, acts, components, elements, and featuresdiscussed in connection with any one or more embodiments are notintended to be excluded from a similar role in any other embodiments.

Also, the phraseology and terminology used herein is for the purpose ofdescription and should not be regarded as limiting. Any references toexamples, embodiments, components, elements or acts of the systems andmethods herein referred to in the singular may also embrace embodimentsincluding a plurality, and any references in plural to any embodiment,component, element or act herein may also embrace embodiments includingonly a singularity. References in the singular or plural form are notintended to limit the presently disclosed systems or methods, theircomponents, acts, or elements. The use herein of “including,”“comprising,” “having,” “containing,” “involving,” and variationsthereof is meant to encompass the items listed thereafter andequivalents thereof as well as additional items. References to “or” maybe construed as inclusive so that any terms described using “or” mayindicate any of a single, more than one, and all of the described terms.In addition, in the event of inconsistent usages of terms between thisdocument and documents incorporated herein by reference, the term usagein the incorporated reference is supplementary to that of this document;for irreconcilable inconsistencies, the term usage in this documentcontrols.

Having thus described several aspects of at least one example, it is tobe appreciated that various alterations, modifications, and improvementswill readily occur to those skilled in the art. For instance, examplesdisclosed herein may also be used in other contexts. Such alterations,modifications, and improvements are intended to be part of thisdisclosure, and are intended to be within the scope of the examplesdiscussed herein. Accordingly, the foregoing description and drawingsare by way of example only.

What is claimed is:
 1. A sludge digester comprising: a vessel having asidewall and an interior volume configured to receive and containsludge; a cover configured to extend over the interior volume of thevessel and seal the interior volume from the atmosphere and trap a gasgenerated by sludge contained in the vessel, the cover comprising: aframe structure constructed and arranged to form: a skirt member formedat a periphery of the cover extending downwardly into the vessel; and acontinuous ballast ring attached to a lower portion of the skirt memberand configured to form a trough member with an interior surface of theskirt member, the trough member having a closed bottom end, an innerwall extending upward from the closed bottom end, an open top end, andone or more brace elements extending between the skirt member and theinner wall; and a guide system coupled to the sidewall and the skirtmember and configured to allow vertical displacement of the cover withchange in volume of at least one of the gas and the sludge contained inthe vessel beneath the cover.
 2. The sludge digester of claim 1, whereinthe frame structure includes a series of frame sections, each framesection forming one of a chord or a curve that connect to each other todefine a generally circular outer perimeter of the cover.
 3. The sludgedigester of claim 2, wherein each curved frame section has concavelycurved radially outer and inner walls and each chordal frame section hasa concavely curved radially outer wall and a planar inner wall.
 4. Thesludge digester of claim 1, wherein the trough member is configured tohold a ballast material.
 5. The sludge digester of claim 4, wherein theballast material is a continuous solid concrete ring.
 6. The sludgedigester of claim 1, wherein the cover further includes a gas domedisposed in the center of the cover and extending axially downward belowa lower portion of the skirt member.
 7. The sludge digester of claim 6,wherein the frame structure is further constructed and arranged to forma series of radial beams, each radial beam extending from an upperportion of the skirt member to the gas dome.
 8. The sludge digester ofclaim 7, wherein the cover further comprises at least one cover plateattached to the frame structure and configured to extend betweenadjacent radial beams.
 9. The sludge digester of claim 1, wherein theframe structure is constructed from steel.
 10. The sludge digester ofclaim 1, wherein the frame structure further includes an inlet openingfor delivering ballast material to the trough member.